The basic principles and techniques for electromagnetic (EM) logging for earth formations are well known. For example, induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, a transmitter transmits an electromagnetic signal that passes through formation materials around the borehole and induces a signal in one or more receivers. The amplitude and/or phase of the receiver signals are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom are recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used by analysts.
EM techniques are used in logging-while-drilling (LWD) and wireline logging. Such techniques are used to evaluate formation parameters, including formation resistivity, formation anisotropy, boundary position, etc.
A parameter (such as resistivity) of a given formation may be isotropic (equal in all directions) or anisotropic (unequal in different directions). Measuring such a parameter in an unconventional formation, containing for example fractures and/or folds, is a challenge.